Electronic surface inspection system employing full frame scanning



Nov. 24, 1964 J. F. LAYCAK ETAL ELECTRONIC SURFACE INSPECTION SYSTEM EMPLOYING FULL. FRAME SCANNING 3 Sheets-Sheet l Filed NOV. 2l, 1961 Nov. 24, 1964 .1. F. LAYCAK ETAL. 3,158,748 ELECTRONIC SURFACE INSPECTION SYSTEM EMRLCYTNC FULL FRAME SCANNINC Filed Nov. 2l, 1961 3 Sheets-Sheet 2 B f-Bvlem J\ f\ J\ j\ C 1 1 j D j D 1 1 1 1 1 1 I l l l l 12e |32 l J r 1 1 1 1 1'50?1f'521 I1 l1l 1 l1H K 1sof112a4I/2ms1 1 1 1 1 11 1 11 1 11 L 30d 1-l281 1 111 111 1J M V L L V L m N '30H 1 1 50N 1|52 1 1 M FIC-1.5

INVENT ORS John F. Loycok Reginald D.Langrr1on 81 Angelo Domme Nov. 24, 1964 1. F. LAYcAK ETAL ELECTRONIC SURFACE INSPECTION SYSTEM EMPLOYING FULL FRAME SCANNING 3 Sheets-Sheet 3 Filed Nov. 2l, 1961 United States Patent Office No., ,4, a

This invention relates to a system adapted to inspect the surface of material for imperfections and quality where the imperfections have a different optical appearance than the remainder of the material as, for example, where the imperfections are discernible by differences in color, emitted light, reected light, or combinations of these. More particularly, the invention relates to a surface inspection system of the type described wherein an electron-optics device utilizing full frame scanning is employed to inspect the surface of moving material.

rihe present invention is based upon the fact that in most materials the surface defects have a different optical appearance than the remainder of the material. For example, in the case of a reiiective surface, defects will generally' have a lower light intensity and appear darlter than the remainder of the material when light is directed thereon. Similarly, in t le case of an irradiant hot object, defects will appear brighter or darker than the body of the material. These diierences in intensity may be converted, by means of a photosensitive device, into electrical current variations which, in turn, may be used to indicate the presence of a defect, or for control purposes. if the defects have a different color than the remainder of the material, suitable iilters may be employed whereby only a particular color or colors will activate a photosensitive device to produce an electrical current variation with the same result as that achieved with an irradiant or reilective surface.

In the past, systems have been devised for inspecting the surface of moving material wherein an electron-optics de lice, such as a vidicon, is employed to scan an image of the surface of the moving material with an electron beam along a straight-line path extending transverse to the direction of movement of the material, the arrangement being such that each time the electron beam interects the image of a defect which has a different optical appearance than the remainder of the material, it will produce an output pulse. lf the scanning frequency varies as a function of the speed of the moving material, or if some other type of speed compensation is employed, an indication of the number of defects on the material may be obtained by counting pulses at the'output of the electron-optics device.

One di'iculty with single line scanning systems of the type described above is that the photosensitive surface in an electron-optics device such as a vldicon has certain lag characteristics, meaning that once the electron beam sweeps across the photosensitive surface, a charge is produced on that surface which takes a period of time to decay bach to the point where the photosensitive surface is again at its quiescent or normal state. ln single line scanning, the electron beam is repetitively swept back and forth along the same line, with the result that the charge on the surface never falls to its quiescent state, but is maintained at a higher voltage. -As a result, any charge on the signal plate due to a defect on the surface of material being inspected will have a lo'tver amplitude with respect to the charge intained on the surface by repetitive scanning along a single line than it ordinarily would if the surface charge were given an opportunity to decay as it is, for example, infull frame scanning. Consequentl it becomes dimcult to distinguish defects from the remainder of the surface of the maerial with single line scanning, especially when the maerial being inspected is moving at high speed and under conditions where the defects vary only slightly in optical appearance from the remainder of the surface of the material.

As an overall object, the present invention seeks to provide a new and improved surface inspection system which overcomes the above and other disadvantages of prior art systems.

More specilcally, an object of the invention is to provide an electronic surface inspection system for moving material wherein full frame scanning of the photosensitive surface of an electron-optics device is ernployed to eilect improved sensitivity in detecting defects and to vitiate the limitation of slow response photosensitive pickups relative to the high speeds at which the product to be inspected travels.

Another object of the invention resides in the provision of a surface inspection system for moving material in which the output of the system will always be the same for a given surface imperfection condition regardless of the speed of the material being inspected.

Still another object of the invention resides in the provision of a surface inspection system which will individually inspect the surface of successive lengths of material and produce output signal whenever the character of the defects in each individual or successive length exceeds a pre-determined condition.

in accordancewith the invention, hereinafter described in detail, the material to be inspected travels over a conveyor. Disposed above the conveyor is an electronoptics pickup device, such as a vidicon, adapted to scan the surface of the material traveling over the conveyor with an electron beam. The vidicon or other photosensitive device operates on a full frame scan. That is, the electron beam of the photosensitive device moves bacic and forth transverse to the direction of the movement of the material as well as parallel thereto, the transverse scanning frequency being much higher than the scanning frequency parallel to the direction of movement. In this respect, the transverse scan corresponds to the horizontal scan in a conventional full frame TV camera tube; whereas the scan parallel to the direction of movement corresponds to the vertical scan of a conventional Tv tube. lf the material being inspected is hot and irradiant, defects on its surface will appear brighter or darker than the remainder of the material and, thus, will have a dierent optical appearance. Similarly, if a reective material is being inspected such as tlnplate or stainless steel, the surface of the material within the field scanned by the photosensitive device may be illuminated by external means. In this latter case, defects appearing on the surface of the reflective teri l will have a lower reflectivity and appear darlte the material being inspected is such that defects have a dilerent color than the remainder of the material, suitable iilters may be disposed between the surface and the electron-optics pickup device whereby a diilerent optical appearance of the defects, as they appear to the pickup device, will be achieved. in anyv of 'these cases, the electron beam ot the photosensitive pickup device will scan across the surface of the material being inspected to produce a video signal in which a relatively long pulse is produced for each 4transverse or horizontal scan of the electron beam, this pulse being produced as the beam scans across the width of the illuminated or irradiant surface of the moving material. Superimposed on this long pulse are short pulses which arise when the electron beam scans over theY iinageof defects of different optical appearance, the widthof each pulse being proportional to the width, along the scanning line, of the Y nal.

Athe material regardless of its speed.

' numberL of the defect pulses proportional defect it represents. The video signal, after suitable shaping, is differentiated to produce a short spiked pulse of one polarity at one edge of each pulse in the video signal and a short spikedpulse ofthe opposite polarity at the other (edge oi'eachfpulse in the original video sig- By eliminating the spiked pulses of one polarity, only a singlespikedpillse will remain in the resultant signal foreach pulse in-the original video signal. Thereafter,"by"eliminating, the spiked pulse due to the relatively long pulse produced when the electron beam scans over an iniagefof the surface being inspected, only those spiled pulses' due' todefects will remain; and by counting the'f're'maining spiked pulses, an indication of the condition ofthe surface ofthe material being inspected will be. obtained: v

'It will be'apparent that in order to obtain an accurate indication; ofthe surface condition of the material being inspected, the'Y electron beam 4shouldscan over each incre'nnental lengthoffthe material only once. Y Accordingly, the Vertical scanning frequency ofthe electron beam (i.e., that"parallel'to"the direction of movement of the material) must lvary as a function of the speed of the materialbeing inspected. Furthermore, Vassuming that thefver tical 'scanning/frequency does vary as a function of speed,- a scan velocity greater than the surface vvelocity 'oflthe material being vinspected must be employed, the scan'velolcity preferably being Vtwice that Y ofthe velocity of the material 'being inspected in order Y' toV obtaincomplete inspection of the surface.

It willalsfo b'ev seen" that iflthre transverse or hori. zontall scanning frequencyffor theelectron beam is constant,` the 4total number'of defect pulses produced for any given length,V ofthe material will bea function of its speed. Th'atfis, ifthe'material travels along its conve'yor at a highfrat'e of speed, "a'fewer number of defect pulses will be produced during one foot of travel than would be produced for the same foot of travel if the speed of the body'were decreased. Since it is desired to obtain anindication of the number of vdefects over a given length' of the bbdy, and since the number of defec pulses is dependent upon the speed of the body, some means must' be' provided toinsure that the number of defectpulses will be the sarde for a particular length of Otherwise, the output of the system`will not be a true' indication of the totalnumber'of defects.

Accordingly, in one ,embodiment of the invention'described herein', speed' compensation is achieved by regulating'the transverse or horizontal sweep repetition rate of the electron beam of the photosensitive pickup device whereby itis proportional to the speed of the material being inspected. In'thismanner, the defect pulses will always lbethesame"regardless of the speed of the Inaterial.V 'lnanother embodiment of the invention, a gatingl signal'is produced having a number of pulses therein proportional to the speed' of the material being inspected.

By'g'atingthe defect vpulses atfthe output of the photosensitivepickup device vwith this Vgating signal, only a of. tl1 e"body` willremain.V Thus, if the material is traveling rapidly/ agreater 'ntmiber of defect pulses will be fed to a 'counteror 'therfsirnilar device overa given time intervaljthan Will beffed over the same timeinterval Vwhen. the` yspeed-"off the 'material is decreased. In this manneilthe'nu'rnber of'defect'pulses fed to Athe counter will always`be the saine forl argiven defect condi-'fion re- Y gardless of speed. t

Themabove and other-objects and features of the inventionV willjbecome` apparent from the following detailed description taken in connection with the accompanying drawings which tion,land inwhich: i l

FIGURE l is` a'block schematic circuit diagram of4 available A'transverse or horizontalrr sweep repetition to the speed formrga part lof Y this 'specil'nza-n one embodiment of the'present inventori which employs 4 frequency to compensate for variations in speed of the material being inspected;

FIG. 2 is a schematic illustration of the scan eilected by the circuit of FlG. l;

FIG. 3 illustrates wave forms appearing at various points in the circuit of FIG. l;

FIG. 4 is a schematic block diagram of another embodiment of the invention wherein the transverse or horizontal scanning frequency is constant, and wherein speed compensation is achieved by means of gating pulses having a recurrence frequency proportionalto the speed of the material being inspected; and

FIG. 5 is an illustration of wave forms appearing at various points in the circuit of FIG. 4.

Referringnow to FIG. 1, the object'being inspected is illustrated as a strip of material 10, such as reflective tinplate or stainless steel, which moves along conveyor rolls 12. As will be understood, a Wideband across the strip l@ is illuminated by means, not shown, the illuminated area beingl at least equal to that scanned by an electron-optics device such as an electrostatic vidicon ld positioned above the strip 1li/. Although the material shown hereintas being inspected is reliective, it should be understood that the material may be an irradiant steel slab, the important thing being that the obgect has a different light intensity, or at least a dilerent full frame optical appearance, than the background surrounding it. Y

cylindrical in shape, extends for most of the length of the tube and is connectedv toa screen 30 situated near, the photoconductive target 20. The targetZ ycomprises a light sensitive element consisting of a thinlayer of photoconductive material'disposed onQa signal electrode. As

will-be seen, ythe photoconductive materialv shouldv bel such' as tohave very littleV photoconductive lag, meaning that a signal will buildup on the material as anele'ctr'on beam sweeps thereacross in a short period of time with a minimum amount of delay or lag. Disposed Within the vidicon are a pair of vertical electrostatic deilectionplates 32 and apair of horizontal electrostatic deilectionplates 34. TheplatesSZ are-at. right angles to plates 34, itV

being understood that onlyone of such plates 34 is shown in FIG. 1 due to the fact that thevidicon is Yshown in cross section, the Aother horizontal .leiilectiony plate :Mbe-

ing disposed above theplaneV of the drawing andi-parallel thereto.. Y Referring now to FIG. 2, the scanning eld lof the electron-optics device 14., is shown as extending across" the VWidth of the materialliland along the'length thereof for a distance dp VBy applying .la high scanning frequency to the horizontal deliection plates 34 while applying a lower` scanning frequency to the vertical deflection plates 32,` the electron beam will scan VbackV and fQrth across the width of the material ltlmany times as it scans along the distance d1 once. Y of` travel'of the beamris schematically illustrated'bythe dotted line in FIG. 2,H Y I practice thew-spacing'between the transverse scans will-be muchless than that illustrated due to the fact, that the transverse or horizontal scanning frequency is onl the order of 260 times that of the vertical scanning frequency; As the electron beam scans over sect defects'on the surface of Vstrip 10, which defects are numbered 3,{38;and 4l). Furthermoreas will herein-v after be seen, each time the electron beam 'intersectsa defect 35,138'or 4t) it willproduce a pulse in its kvideo output signal to VVindicate"y the existence of they'defect.

inthe The general back-and-forth path it being understood thatin actual 'its fleld,' it will interanaemia material it) while advancing forwardly and terminates at point lid, at which time it dies bach to point 42 and repeats the cycle. ln order to give a true indication of the condition of the surface of strip lil it is, of course, necessary to scan over each defect once, and only once;

and this is controlled by the vertical scanning velocity` of the electron beam. In order to scan each defect only once, the vertical scan velocity should be approximately twice that of the strip lll in the direction of arrow Thus, if it is assumed that defect 3o has just entered the field of scan from the right as viewed in FIG. 2, it will be scanned by the electron beam which moves to the left at twice the speed of the strip it) in the direction of arrow 46, while scanning back and forth across the width of the strip, and will reach point 44 after the defect 36 has traveled one half the distance dl, this being due to the fact that the speed of the material lll and defect is one half that of the vertical scanning velocity of the electron beam. As soon as the beam reaches point 44 it will repeat the scanning cycle from point 42, moving across the distance d1 in one half the time required for the defect 36 to move thereacross. By the time the electron beam again traverses the distance d1, the defect flo will have moved out of the field of scan so that it is not scanned again. Thus, any point on the material l@ is scanned only once by virtue of the fact that the vertical scanning velocity is approximately twice that of the speed of the material 1i). if the vertical scanning velocity is increased over a ratio of two to one, then certain areas of the surface will be scanned twice; whereas if the vertical scanning velocity is decreased over the two to one ratio, then certain areas of the surface of material 'lll will not be scanned.

Since the vertical scanning velocity should bear a distinct ratio with respect to the linear velocity of the material l0, it can be seen that the vertical scanning velocity must vary as a function of strip velocity. Also, the number of pulses produced in the video output signal of the vidicon due to defects will be a function of both the horizontal scanning velocity and the linear velocity of the material 10. That is, if it is assumed that the horizontal scanning velocity is fixed md that the electron beam sweeps back and forth across defect 3d one thousand times, for example, while the strip or material l() is moving at one speed, then the electron beam at the same horizontal scanning frequency will sweep back and forth across the defect 36 only 500 times if the speed of the material it) is doubled. Thus, the horizontal scanning frequency must also vary as a function of the linear velocity of the material it?, or some other type of speed compensation, such as the gating arrangement hereinafter described, must be employed to insure that the number of efect pulses at the output of the electron-optics device 14 is the same for a given defect condition regardless of strip speed.

Circuitry for effecting full frame scanning wherein both the horizontal and vertical scanning frequencies vary as function of the speed of the material fill is shown in FlG.

Vl. Two tachometer generators 4S and Sil are connected to the conveyor rolls i2. Generator dit is connected to a variable gain amplifier 52 as well as an automatic gain control circuit 54 which controls the gain of amplifier 52 in theV usual manner to produce a constant amplitude sine wave output regardless of variations in input amplitude due to changes in the speed of strip lil. With reference to PEG. 3, the output of the tachometer generator S appears as wave form A and is the usual sine wave.

After passing through clipper Se, :the negative half cyclesV of the sine wave are removed so that the wave form now appears as wave form B in FIG. 3. The wave form is then passed through amplifier Sti to a Schmitt trigger circuit dil. As is well known, the Schmitt trigger circuit is a multivibrator having two electron valves therein, only one of which conducts at any one time. brator may be switched from one of its two conducting The multivistates to the other whenever an input signal applied thereto exceeds a predetermined voltage level, such level being indicated by the dotted line el on wave form B. Thus, the wave form at the output of the Schmitt trigger 6i) appears as wave form C. This wave form is differentiated in dierentiator e?. to produce wave form D in FlG. 3 wherein a sharp spiked pulse is produced at the leading and trailing edge of each of the pulses in wave form C, the spil-red pulses at the leading edge of each pulse in wave form C being of positive polarity while the spiked pulses at the trailing edge of each pulse in wave `form C are of negative polarity.` The differentiated pulses are passed through amplifier 64 to a phantastron circuit 65 which will be triggered by each of the positive spiked pulses in wave form D to produce a series of sawtooth coniigurations such as that shown by wave form E in FIG. 3. The sawtooth wave form E is passed through clamp 66, amplifier 63 and horizontal output circuit '70 to the h rizontal deflection plates 34. Thus, wave form Eis that which causes the electron beam of the vidicon to sweep bacl; and forth along the horizontal or transverse dimension shown in 2. The scanning frequency in the horizontal direction is relatively high, being on the range of about l5,GGO to 16,0% cycles per second. The RC time constant of the phantastron circuit is adjusted to maintain a constant scan time for each of the sawtooth configurations in wave forni E, however the recurrence frequency of the wave form is a function of the frequency of wave form A which, in turn, is a function of the speed of strip il?.

The sawtooth output wave fornrof phantastron 65 (wave form E) is also passed through a Schmitt trigger circuit 72 and cathode follower 7d to a phase inverter 76. rihe ,output of the phase inverter will appear as wave form F in FiG. 3 wherein a negative pulse persists during the nybaclr time of the electron beam in the horizontal direction. rThe negative blanking pulses in wave form El are applied through blanlring output circuit 78 to the control grid 25 of the vidlcon lll whereby the electron beam is cut off during the persistence of a negative pulse in wave form F. Alternatively, the pulses in wave form F could be positive, rather than negative, and applied to the cathode 22 with the same overall effect.

it can be seen that if only that circuitry already described were employed, the electron beam would scan baclfand forth across an image of the surface of material i@ along a straight-line path. In order to effect full frame scanning wherein the electron beam scans in both the horizontal and vertical directions, it is necessary to apply a sawtooth wave to the vertical deilection plates 32, the frequency of this latter wave being much lower than that applied to lthe horizontal deflection plates 34. In the present instance, it will be assumed that the vertical scanning frequency is in the range of about 6G cycles per second. Accordingly, the output of ampliiier S8, which is amplified wave form B, is fed to a frequency divider 89 which reduces the frequency to about 60 cycles per second, the exact frequency being a function of the speed of strip lil. From frequency divider Si?, the signal passes through a phase inverter 62 and amplifier S to differentiator Se which will produce an output signal similar to wave form D in PIG. 3, but wherein the frequency of Vthe.

spiked pulses is greatly reduced.- The differentiated pulses, after passing through clipper fidare fed to a oneshot multivibrator all which produces a blanking output pulse to circuit 7d for the vertical scan. The blanlringV pulses from multivibrator 9d are similar to those shown in wave form F of FIG. 3 with the exceptionthat they re reduced in frequency.

The output of the one-shot multivibrator 9?, being pulses which occur at a frequency of 6G cycles per second,

about GQvcycles per second 'as contrasted with the output of phautlastron which is at a frequency in the range of Vabcnlt15,609, to 16,0,00cycles per second. Since the phant'astronQi, in the iirst'instance, iscontrolled by the ltachoirneter' generatormthe frequency of its output sawtoofth yWave form'willalso vary as a function of the speed of strip 10, However, as the frequency of the sawtooth waveform varies, the slope of the sawtooth will also vary as Well as its n'iaxirnurnV amplitude. Consequently, in yorder to( insure that the electron beamwill always sweep through the full distance d1 shown in FIG. 2, some means must be provided to ,insureA that the amplitude of the.. sawto'othhrwave fo'rmapplied to the vertical deflection plateswilli always be thesanle regardless of its frequency. Forthis purpose, theloutput of tachometer generator 48 is appliedjo arfrequency sensor V96the output of the frequeu'cy'sensorfbeing'applied toareetiiier 98 and lter 100`A Whichwapplie's thenrectifled voltage to a variable slope circuit 192 which compensates for variations in the slope andA amplitude at the of phantastron 94 in response to changes Yiufrequency.Y Thus, theV output of'circuit 162 is'a sawtoothwave form of lconstant amplitude regardlessofgfrequency. This Wave form is applied through amplifier 164 and vertical output circuit i3d to the'vertical deectiou plates 32 inthe vidicon 14.

'Tlievideo outputrwave form on signal plate 2G of vidicon 14 appears on lead 198. This Wave forni, illustrated as vvavefform Guin. FIG; 3,Y is a series of relatively long pulses having superimposed thereon short pulses indicatiugjthe `presence of defects. it willv be apparent from- Wave form G )that Yas Vtheelectron beam sweeps from one side of the` onveyor to the other, it will first scan thedark background portion of the conveyor until it reaches point afvvhicliisdthefedge Vofthe strip or other material ill. Afterintersectingsthe edge or" the strip, the voltage of the video Wave formincreases because of the greater light in teljisillyoffthestrip.` AtV point b, theelectron beam intersectsft'hev defect 36 shown in FGIZ, for example. This defectlias a ,lesser reilectivity than the remainder ofthe str ipE and,rconsequently, alnegativc pulse llt) is produced inthe Waveform. Finallypat point c in the Wave form the electron bearn leaves'the edge ofthe strip and the voltagejdecreases due to the dark background of the conveyor.I Between points 'd and e, the electron beam returnsv to its originalstarting vposition over a very short intervalof Vtirnegpand the cycle is repeated.y As the electronbeam continues to scan in the verticaldirection, it Wiiljin'terisect th'edef/ectsSS and e@ many cycles later.

Since.thefdefeotsfw33` il are aligned across the widthVV ofrthelstrigthey Will produce two pulses i12 and il@ ink thevideowave form. The pulses liti, H2 and 1L@ will repeat in the lWaveY forni for as many times as the defects which theyv iepreisentY are scanned by vthepelectron beam, however each defect will ybe scanned only once, as was eXp-la ed above`,rduelto Athe `fact that ythe vertical scanningV velocityvis twice :thellinear speed of strip iii.

AR-eve'rting again to l', from lead the Wave forniiis passed tliroughpa lirst clipper li and a irst amplifier 118` fo a'second clipper i2@ and a second ampli- ,lier'122l The function of the VAfirst clipper lle is to remove theflovve'rpo'iftion'` of the video Wave form G so that lonly'therpulse"bejtvlleen' points a kand c remains.V Circuit pulses i3d andl 132 are'vthosel due to ,the defect pulseliill.

W ave form l at the `output ofdiiferentiataor ilZfifis thereafter passed through ampliier, E27 to a clipper 129 which removes the positive pulses Quand i3@ in'diifverentiated Wave form J to produce Wave form kK wherein only the negative sharp pulses ril. and 123 remain. 'These pulses arelinverted in -arnpliiier 131 and passed through,cathodeY I follower 133 to a delay Yline 134 which delaysthe pulses by one-half microsecond to producen/ ave forrnL This wave form, after passingrthrough amplier livisfed to gatecircuit li. Y

Reverting againvtothevidicon 14, Vits output video Wave form G on lead Mld is ,also passed through'ampiiiier 1431 toa Schmitt trigger `cir-cuit 142, The voltagelevelat which the circuit 142 will switch vfrom one stable` state to the `other is indicated at i634 ,on Wave form G inl FIG. 3. Thus, the output voltage, of theSchmitt trigger Will rise at the leadin7 edge of each relatively long pulse or pedestal in WaveforrnG and will fall at itstraiiingedgetopro ducepa series of pulses appearing as Wave form M FIG, 3. Each of thepulses in Wave form M, it. Willbe noted,j have .the sainepulse Widthsandphase positionA as a corresponding pedestal in Wave forni G.V These pulses are passed through ampiilier idd togate circuit. 13S. By gating Wave form.. L from amplier 136 vwith .waveform M Vfrom amplifier 146, only those pulses in Wave form L which coincide with a pulse in wave form M will' aplaufk at the output of gate circuit i3d. From an examination of FIG. 3, irtNWill benoted that since wave form K was delayed by one-half microsecond to produce Waveforurls, the pulse lf due tothe trailing edge of the pedestalnin the original videorwave formx does not coincide with av pulse inwave forinM. Consequently, pulseY 128 is eliminatedjin the gate circuit l .whereby vwave form N isapplied to a counter flithis Wave form containing only the pulses 13) which were produced 'by the defectl 36v shown in FIG. 2. It is, therefore, apparent that a single` spiked pulse Will'appear at the output of gate lcircuit Y138 118 a'mpliiiesl lthey/ave form and clipperV 129 refines the Y Y 'p clipping action sonas toi'nsurethat clean pulses of amplitude'appear *atF the output'gof'anapliier 122. These* pulses;then,Lappear asrwave'form Hv in FlG. 3 and are passed Vto'adiieifentiator.124.."The,voltage wave forza 'Y appearing `at the output of Yfthe ditierentiator xvillb'eza seriesofsharpvoltage pulses which occur 1n time at the Ypoints whereythetinputisquare Wave signal changes from Y one''volta'ge level tofanotherz f y'This Waveform 4is Villusr- I trated as Wave formi in.FlG, '3. As theV input voltage j Y' y Y Y speedA of the materialrlti. Speed cornpensation,lilou/ever,

' changes/mina positive direction,` the` sharp pulse produced bythe diferentiator Vis also positive, while a'negative sli` p for each defect scanned by the electron beam of the camera tube( i4. Y in a similar manner, the pulses llZand 111iv in the original vvideo Wave forni G produce spiked pulses 3.52, and these only arepassed Vtluough'the gate circuitlwhile the spiked pulses due to the leading Yand, trailingedges of the pedestal arerelirninated. Thus, thev output of counterled will be, an indication fof the number of defects on the surface of the strip 1li, and sincethe frequencies of the vertical and horizontal scans vary as a function of the speed of the material, the number of pulses arriving at counter 44 for a given defect conditionwill always be the saine, regardless of the speed of the ma? terial. Y

If it is desired to determine the defect condition on successive equal'increments of the material lil, the output of Schmitt triggerV oily/hiel; is IWavegforrn'l2 in FIG'. 3 may be passed through a one-shot multivibrator 154i (FlGn' l) to a `counter i553; the output of the countern 1565 beingv a". pulse on lead ESS each time the material l@ has traveled Y throughra 'predetermined distance.` This pulseftliemis,

Y Asrwill beV understood,speeld compensation rinthe cir- V 4cuit of FlG. l Was achieved by vrendering the'horizontal sweep frequencyvof Ythev vidicon `14 proportional to thev may also be derived'by the circuit shownn FIG.` 4.V

9 to those already shown and described in connection with FIG. 1, elements in FIG. 4 which correspond to those of FIG. 1 are identiiied by like reference numerals and are not hereinafter described in detail. The major difference between the circuit of FlG. 4 and that of FIG. 1 is that the output of amplifier 58 is not used to produce the horizontal scanning frequency. Rather, the horiontal scanning frequency originates with a pulse generator l@ which produces a pulsed output of constant frequency. This pulsed output is applied to phantastron 164 whic produces the sawtooth output wave from A shown in FIG. 5. This wave form, unlike wave form E in FIG. 3 is of constant frequency and does not vary in irequency as a function of the speed of the material 19. The Wave form A is passed through ampliiier E66 and horizontal output circuit 158 to the horizontal dedection plates 34 of the vidicon 14.

The output of pulse generator 152 is also applied through cathode follower 17@ and phase inverter 172 to the blanlring output circuit 78. The pulses from genrator 152 thus comprise the blanking pulses applied to the control grid 26 of the vidicon 14 for the horizontal sweep.

The circuitry at the output of the vidicon i4 in FIG. 4 is identical to that shown in FIG. l except that an additional amplifier 174 and gate circuit lfi are interposed between gate 138 and the counter 149, The output o the gate circuit 13S in FlG. 4 may, for example, appear as wave form B in FIG. wherein two spiked pulses 17S Aand 186 are produced each time the electron beam sweeps across the width of the materid lli. Furthermore, the pulse 17S and 18d will occur repetitively at a iixed frequency as long as the electron beam sweeps across the defects. Since the horizontal sweep repetition frequency does not vary in the circuit of Flf. 4, the number of spiked pulses appearing in wave from B' will he a function of the speed of the strip. That is, when the speed of the strip is reduced, the electron beam will scan hack and forth across the defects for a longer period of time than it would if the speed of the strip were increased.

The output of tachometer generator 5% is shown in FIG. 5 as wave form C' and comprises a sine wave. This wave form, after passing through clipper 55 and amplifier 5S appears as wave forni D in FIG. 5 wherein only the positive half cycles of the wave form remain. The wave form D' is applied to Schmitt trigger circuit 60; and the output of this circuit appears as wave form E which corresponds to wave form C in FG. 3. This wave form is applied to a diierentiator i552 which produces the wave formiF in FIG. 5 wherein a sharp spiked pulse is produced at the leading and trailing edges of each pulse in wave form E', the pulse at the leading edge being positive while the pulse at the trailing edge is negative. This wave form (wave form F' in FIG. 5 is passed through amplier 1&4 to a phantastron 136 and Schmitt trigger circuit 183, the output of the Schmitt trigger circuit being wave form G' which comprises a series of pulses of fixed width as determined by the phantastron ld, with the leading edge of each pulse in wave form G corresponding with the positive spiked pulses in wave form F. The wave form G is then passed through a cathode follower circuit 199 to the gate circuit 176. Thus, wave form E is gated with wave form G to produce wave for H wherein positive spiked pulses are produced only upon coincidence of pulses in wave form G with those in wave form B'. Since the number of pulses in wave form G' (i.e., the pulse recurrence frequency) is proportidhal to the speed of strip 1li, the number of pulses appearing in wave form H will also be proportional to spee1 for a given defect condition.

As long as the electron beam of vidicon ld sweeps over l efects on the surface of strip 1t), spiked pulses Vwill appear in wave form B This wave form will appear regardless of the speed of the strip lll, just so long as the electron bearnscans a defect. The wave form G' at the cutput of cathode follower 1%, however, will be produced only while the strip lll is moving since it depends upon the frequency at the output of tachorneter 5t). Furthermore, the frequenecy of the pulses in wave form G depends upon the speed ot the strip when it is'moving. Thus, if the speed of the strip were doubled over that assumed the wave forms given in FlCf. 5, then six pulses would appear in wave form G rather than the three shown. it is, therefore, evident that if the speed of the strip lll were doubled, the number of pulses in wave form B' which would be passed to gate l76 would also be doubled for the time interval shown; and, in general, it may be said that the number of pulses in wave form B arriving at gate 17d is proportional to the speed of the strip lll. As the strip travels through the distance d1 (FIG. 2), for example, the number of pulses reaching gate 176 will he the same for the given total defect area regardless of the speed of the strip 1li, the only difference being that it the speed of the strip is increased, the time interval required for a given number of pulses to arrive at gate 176 is decreased.

Although the invention has been shown in connection with certain specific embodiments, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

We claim as our invention:

l. ln apparatus for detecting defects on the surface of moving material in which the defects have a different optical appearance than the remainder of the material, the combination of means including an electron-optics device for full frame scanning an image oi' the surface of said moving material with an electron beam wherein the beam scans transverse and substantially parallel to the direction of movement of the material, the output ot the electron-optics device being a pulsed signal in which a pulse is produced each time the electron beam scans over a defeet, and means for counting at least part of the pulses at the output of said eletcron-optics device to obtain an indication of the surface condition of said material.

2. ln an apparatus for detecting defects on the surface of moving material in which the detects have a diierent optical appearance than the remainder oi' thematerial, the combination of means including an electron-optics device for full frame scanning an image ofthe surface of said moving material with an electro beam wherein the beam scans transverse and substantially parallel tothe direction ot movement of the material, the scanning frequency transverse to the direction ot movement being greater than that substantially parallel to the direction of movement, the output of the electron-optics device being a pulsed signal in which a pulse is produced each time the electron beam scans over a defect, and means for counting at least part of the pulses at the output ot' said electronoptics device to obtain an indication of the surface condition ot said material.

3. ln apparatus for detecting defects on the surface of moving material in which the detects having a dierent optical appearance than the remainder of the material, the combination or" means including an electron-optics device for full frame scanning an image of the surface of said moving material with an electron beam wherein the beam scans transverse to and substantially parallel to the direction of movement o the material, the scanning frequency transverse to the direction of movement being greater than that substantially parallel to the direction of movement and the speed of the electron beam along said image parallel to the direction or movement being laster than the speed of the image ot said moving material, the output ot the electron-optics device being a pulsed signal in which n pulse is produced each time the electron beam scans over a detect, and means for countingue pulses at the output ot said electron-.optics opticaljappearance than the remainderV ofthe;

l. l f Y device, to obtainan indication. of the surface condition of said material.

4. apparatus for.l detecting defects on the surface of Vfrequency transverse to the direction of movement being greater-.than that substantially .parallel to thedirectionof movementland thescanning speed ofsaid electron beam parallel Yto the. directionoffmovcment of thermaterial beingy substantially twice thek speed .of the image of Vthe surface ofsaidmovingmaterial, the output of the Velectron-optics .device being .al pulsed signal. in. which apulse isproducedeach time` theV electron beam,` scans over a defect, and .meansffor ,counting at leasta portion of the pulses at the output of said electron-optics` device to obtainan indication of thesurface condition of said material. 5.. Inapparatus for detecting defectson the surface of movingmaterial in which the defects `have a different optical appearancev than .the remainder of the material, the combination. of means. includiugan electron-optics device for full frame scanning an image of thesurface of said moving Vmaterial at a frequency Which varies .as a function ofthespeedofsaid movingmaterial, the output ofthe. electronvoptics device, being a Vpulsed signal in Whicha pulse is produced eachy time the electron beam scansovera defect, andvmeanslfor counting the pulses at the output of said electron-optics deviceito obtain anjndication ofthe surface condition of said material.

6. In apparatus for detecting rdefects von the surface of moving material in which the defects have a different optical appearance than the remainder of the material, the combination of means including-,an electron-optics de-y vice forfull frame scanning an image of the surface of saidmovingmaterial with an electron beam wherein the beam scans transverse and substantially parallel tothe direction of movementrof thematerialat frequencies .which vary 'as a function ofthe speed of said moving material, the scanning frequency transverse tothe direction of movement being greaterthan that substantially parallel to the directionof movement at all times regardlessof the speed of .the movingrnaten'al, the outputy of the electron-optics device being a pulsed signal inwhich a pulse is produced each vtime the .electron beam scans over a defect, and

means. forcounting the pulses atithe output of said elec-Y tron-,optics device to obtain an indication of the .surface condition of said material.

7. In-apparatusfor detecting defects on the surface of moving material in which the ldefects have a different optical appearance than therremainder of the material, thecombination :of means including an electron-optics device for full frame scanning an image of thel surface of said movingmaterial with anelectron beam wherein thebearn scanstransverse and substantially parallel to the .direction of movement of the material at frequencies which ,varyas apfunction of the speed of said moving material, the. Vscanning frequency transverse to the direction of movementr being greater than that substantially vparallel to the direction of Ymovement and the scanning speed parallelA to thedirection of movement being substantially tvvice that of the speed ofthe image of the surface of `Vthemoving material, the .output of the electronf optics devicebeing a pulsed ysignal in .which a pulse .is

produced each time .the electron beam scans over a de-v feet, and .means for counting the pulses `at the output of Vsaid-electron-optics deviceto obtain Yan indication of the surface .condition of saidY material. o

u .8.y lin-apparatus foridetecting ldefects lon the surr moving material in whichpthe defects have Va different the combination of S means including an Veleintron-'optics ace off device for full frame. scanning'an image of the surface of said moving material with an yelectron bearnwilcteirl` l the beam' scans transverse. and substantially parallelto the direction or movement of thematerial, the transverse scanning frequency being fixed, means. forvarying the scanning frecplencyy substantially parallel to ,the directionl of movement as a function .of therspeed of said movingy V pulsed signal to eliminate a portion ofthepulses therein and produce a resultant signal, the portion, of thepulses eliminated beinga function, of thespeed ,ofjsaid material, and a counter forcounting the pulses -in saidvresultant signal. Y

9. The apparatus ofclaim. S andincludinga device responsive to the, speed .of said. moving material forresetting said counter each; time the material. has moved through a predetermined distance.

l0. In' apparatus for detecting defects onv the surface of moving material in which the defects have a different optical` appearancethan the remainderof, the. material, the combination of means including,V an electrostatic. vidicon forrfull frame-scanning an image of the surface of said material, horizontalv and..verticaldeilection plates in said electrostatic-vidicon, meansfor applying a saW- tooth Wave form havinga frequency which 4Vvaries as -a function of the speed-fof'saidfmoving materail to'said vertical deflection plates,Y means for: applyingla sawtooth Wave-formhavingaa frequency which varies as a function of the-speed of -said movingmaterial to said horizontal deflection plates, the output ofthe vidicon being a pulsed signal inwhich a pulse isproduced each time the elec-A tron beamscans over;l thezimage oiga defecgand means, for, counting` at least some of the pulses Vattire-output of said vidicon to obtainyan` indication ofthesur-face con. ditionfof, said material. Y

ll. In apparatus fort detecting defectsron Ithesurface, of moving material in'whichtheidefects haveaditerent optical appearance than. the remainder l,of 'the material, the combination of means including an electrostatic vidi? con for full -frame scanning an image offtheysurfacf. of said material, horizontal and-vertical deflection'platesinv ,Saidrelectmstatic vdicon; arrangedfinfplwes: extending parallel ,andrhori-zontalto' the direction;of movement of,

said material respectively, means for applying a saw-tooth Y tron beam betWeenthever-,tical deflection plates is'rsubf stantially twice that of the speed of saidimovingmaterial, meansforapplying a sawtoothlyvave ,formv having a fre quency Whichfvaries asa function of the speedqofsaid moving materialto said horizontal deflection plates withthe frequency or the sawtoothwave form ,applied to said horizontal, deliectionplates beingfmuch higher than the: frequency of the ;Wave-.form;appliedfto;the vertical der;

. lection plates, the v.output of thevidicon beinga pulsed; Y

signal in which a pulse is Vproduced.each, time the electron4 beam scansA over the image. of, a defect; and means for; counting the pulses at the outputof said vidicon-to. Obtain` an indication of .theV surface condition for` said fmaterial.

l2. .lnappartus fordetecting efects .onthe Surface: of moving material infwhich the Ydefects have ,a different'. optical appearance than thefremainder of; the material, the Vcombination of; means including an; electrostatic.

vidiconVV for full frame scanningV an'image of the-surface-l of said material, horizontalV and vertical dellection plates. 1n said electrostatic vidieon arranged inplanes extend- 'ingV parallel' and-*perpendicular tothe. direction .ofmovement 'of said material respectively, meanspfor. applying.l

a Vsav/tooth* wave Vformfhavingxa frequency which variesas a function ofthe speed of said rnc-ving material to" sald vertical deliection plates, means for applying a saw- 13 tooth wave form having a frequency to said horizontal deflection plates, the output ot the vidicon being a pulsed signal in which a pulse is produced each time the electron beam scans over a defect, means responsive to the s eed or said moving material for gating said pulsed signal to eliminate a portion of the pulses therein and produce a resultant signal, the portion of the pulses eliminated being a function of the speed of said material, and means for counting the pulses in said resultant signal.

13. ln apparatus for detecting defects on the surface of moving material in which the defects have a dierent optical appearance than the remainder of tbe material, the combination of means including an electrostatic vidicon for full trame scanning an image of the surface of said material, horizontal and vertical deflection plates in said electrostatic vidicon lying in planes extending parallel and perpendicular to the direction of movement of said material respectively, means for applying a sawtooth wave form to said vertical deflection plates, means for applying a sav/tooth Wave form to said horizontal detlection plates, the frequency of the Wave form applied to the horizontal deflection plates being higher than that applied to the vertical detlection plates, the output of the electron-optics device being a pulsed signal in which a pulse is produced each time the electron beam scans over the image of a defect, and means for counting the pulses at the output of said electron-optics device to ob- 14 tain an indication of the surface condition of said material.

14. In apparatus for detecting defects on the surface of moving material in which the defects have a different optical appearance than die remain er of the material, the combination of means including an electron-optics device for full frame scanning an image of the surface of said material With an electron beam to produce a pulsed output signal in which a pulse is produced each time the electron beam scans over a defect, means for counting at least part of the pulses at the output of said electron-optics device to obtain an indication of the surface condition of said material, and a device responsive to the speed of said moving material for resetting said counter each time the material has moved through a predetermined distance.

References Cited in the tile of this patent UNITED STATES PATENTS 

1. IN APPARATUS FOR DETECTING DEFECTS ON THE SURFACE OF MOVING MATERIAL IN WHICH THE DEFECTS HAVING A DIFFERENT OPTICAL APPEARANCE THAN THE REMAINDER OF THE MATERIAL, THE COMBINATION OF MEANS INCLUDING AN ELECTRON-OPTICS DEVICE FOR FULL FRAME SCANNING AN IMAGE OF THE SURFACE OF SAID MOVING MATERIAL WITH AN ELECTRON BEAM WHEREIN THE BEAM SCANS TRANSVERSE AND SUBSTANTIALLY PARALLEL TO THE DIRECTION OF MOVEMENT OF THE MATERIAL, THE OUTPUT OF THE ELECTRON-OPTICS DEVICE BEING A PULSED SIGNAL IN WHICH A PULSE IS PRODUCED EACH TIME THE ELECTRON BEAM SCANS OVER A DEFECT, AND MEANS FOR COUNTING AT LEAST PART OF THE PULSES AT THE OUTPUT OF SAID ELETCRON-OPTICS DEVICE TO OBTAIN AN INDICATION OF THE SURFACE CONDITION OF SAID MATERIAL. 